This invention relates generally to nuclear reactors and, more particularly, core support for F-lattice cores in nuclear reactors.
A known reactor pressure vessel (RPV) of a boiling water reactor (BWR) has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide is spaced above a core plate within the RPV. A core shroud, or shroud, surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The top guide includes several openings, and fuel bundles are inserted through the openings and are supported by the core plate. The core plate includes a flat plate supported by a plurality of beams.
A plurality of openings are formed in the bottom head dome so that components, such as control rod drive assemblies, can extend within the RPV. As an example, for a control rod drive assembly, a control rod drive housing, for example, a tube, is inserted through the bottom head dome opening and a control rod drive is inserted through the control rod drive housing. The control rod drive is coupled to a control rod to position the control rod within the core.
A nuclear reactor core includes individual fuel assemblies that have different characteristics that affect the strategy for operation of the core. For example, a nuclear reactor core has many, e.g., several hundred, individual fuel bundles that have different characteristics. Such bundles preferably are arranged within the reactor core so that the interaction between the fuel bundles satisfies all regulatory and reactor design constraints, including governmental and customer specified constraints. In addition to satisfying the design constraints, since the core loading arrangement determines the cycle energy, i.e., the amount of energy that the reactor core generates before the core needs to be refreshed with new fuel elements, the core loading arrangement preferably optimizes the core cycle energy.
In order to furnish the required energy output, the reactor core is periodically refueled with fresh fuel bundles. To optimize core cycle energy, the higher reactivity bundles may be positioned at an inner core location. To satisfy some design constraints, however, higher reactivity bundles generally are positioned at an outer core location. The most depleted fuel bundles, i.e., the bundles with the least remaining energy content, are removed from the reactor. The interval between refuelings is referred to as a cycle of operation.
During the course of the cycle of operation, the excess reactivity, which defines the energy capability of the core, is controlled in two ways. Specifically, a burnable poison, e.g., gadoliia, is incorporated in the fresh fuel. The quantity of initial burnable poison is determined by design constraints typically set by the utility and by the NRC. The burnable poison controls most, but not all, of the excess reactivity.
Control rods also control the excess reactivity. Specifically, the reactor core contains control rods which assure safe shutdown and provide the primary mechanism for controlling the maximum power peaking factor. The total number of control rods available varies with core size and geometry, and is typically between 50 and 269. The position of the control rods, i.e., fully inserted, fully withdrawn, or somewhere between, is based on the need to control the excess reactivity and to meet other operational constraints, such as the maximum core power peaking factor.
One known control rod includes a central portion having four radially extending blades. The blades define four fuel bundle channels, and when inserting the control rod into the core, the control rod is positioned so that one fuel bundle is positioned within each channel. Therefore, for example, approximately 100 control rods are included in a reactor having 400 fuel bundles.
To reduce the number of control rods necessary for efficient operation, the core of a nuclear reactor includes a plurality of fuel bundles and a plurality of large control rods. Each large control rod is about two times the width of a conventional control rod and includes four control rod blades extending radially from a central portion and arranged at right angles to each other. The blades define four fuel bundle receiving channels. The core is configured so that the control rods are arranged in a plurality of staggered rows with four fuel bundles in each receiving channel. This configuration is defined as an F-lattice configuration.
In this F-lattice configuration a fuel cell is formed by one large control rod and sixteen fuel bundles. The four blades of the control rod divide the fuel cell into four equal quadrants. The fuel bundles are arranged around the control rod so that there are four fuel bundles in each quadrant of the fuel cell. The core is formed from a plurality of fuel cells. In the F-lattice configuration with the large control rods arranged in staggered rows, each edge of a fuel cell is adjacent to and substantially parallel to a blade of a control rod.
The large control rod in an F-lattice configuration complicates the flat plate and beam support concept of the core plate because of the size of the control rod and the staggered arrangement of the rods. The staggered rod pattern permits very little clearance for the support beams.
Normal coolant flow entering the standard sized BWR fuel assemblies is single phased and slightly subcooled. The flow approaches the fuel support vertically upward and then turns horizontally as the flow enters the inlet to the fuel support. The flow then passes through an orifice that provides the required pressure drop, assuring the correct coolant distribution to low and high-powered fuel bundles. The flow then turns vertical again and enters the lower tie plate of the fuel assembly, being distributed around the individual fuel pins.
For the F-lattice configuration, the flow approaches the entrance to the fuel vertically but must flow directly past core plate support beams for about half of the entrances. The support beams obstruct the coolant flow and create flow separation and bi-stable flow. These abnormal flow characteristics can influence the flow pattern at both the entrance and within the fuel assembly.
It would be desirable to provide a core support arrangement for F-lattice configured cores that provides identical flow entrance conditions for all the fuel assemblies.
In an exemplary embodiment, a core plate assembly for a nuclear reactor includes a plurality of support beams, a flat plate positioned on top of the support beams, a plurality of control rod guide tube openings arranged in staggered rows, and a plurality of fuel supports extending through the flat plate.
The guide tube openings have a cruciform shape and include four slots extending radially from a central portion at right angles to each other. The slots define four fuel bundle receiving areas. Each guide tube opening is sized to receive a control rod guide tube.
Each fuel support includes a coolant flow inlet, a coolant flow outlet sized to receive a lower tie plate of a fuel bundle, and a coolant flow bore extending between the coolant flow inlet and the coolant flow outlet. The coolant flow inlet is offset from coolant flow outlet so that a centerline of the coolant flow inlet is parallel to a centerline of the coolant flow outlet. Each coolant flow inlet includes an orifice plate. The coolant flow inlets are positioned adjacent to a support beam, and the coolant flow outlets are positioned in a fuel bundle receiving area. Each fuel bundle receiving area includes four fuel supports to support four fuel bundles.
The above described core plate assembly provides unobstructed coolant flow inlets and therefore identical flow entrance conditions for all the fuel assemblies.